HTML

html-ind

Indent HTML using 2 spaces.

Reason
HTML tends to nest pretty deeply, and understandably so. Thus, a small indentation size works well in this case.

html-case

Use lowercase for HTML tags, attributes, and other identifiers.

Reason
Staying with all-lowercase for the code elements of HTML allows the actual content, which tends to be mixed-case, to stand out a little better among all of the tags and attributes. Content is king, after all.

html-attr

Don’t include whitespace before and after the = symbol in attributes.

Reason
Eliminating this whitespace helps attributes look like a more cohesive unit.
Example
  <div id="introduction" class="inset">
    1 + 1 = 3 is an interesting design principle
    pertaining to borders vs. whitespace.
  </div>
Anti-Example
  <div id = "introduction" class = "inset">
    1 + 1 = 3 is an interesting design principle
    pertaining to borders vs. whitespace.
  </div>

html-mvc

Be cognizant of deprecated HTML elements and attributes. Some of them are like that because they violate MVC.

Reason
The modern view of HTML is that it holds the content or model of a particular web application. Elements like font and center or attributes such as color pertain to the presentation or view, which today is rightly separated into CSS.
But They Still Work!
Not surprisingly, the aforementioned elements (and more) are no longer part of HTML—but you might think they are because many web browsers still “forgive” their use. Accept no mercy.